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Bulbasaurus (meaning "bulbous reptile") is an extinct genus of dicynodont that is known from the Lopingian epoch of the Permian period of what is now South Africa, containing the type and only species B. phylloxyron. It was formerly considered as belonging to Tropidostoma; however, due to numerous differences from Tropidostoma in terms of skull morphology and size, it has been reclassified the earliest known member of the family Geikiidae, and the only member of the group known from the Tropidostoma Assemblage Zone. Within the Geikiidae, it has been placed close to Aulacephalodon, although a more basal position is not implausible.
Bulbasaurus was ostensibly not directly named after the Pokémon Bulbasaur, but rather after its nasal bone bosses, which are unusually bulbous among geikiids; however, the describers noted that the similarity in name "may not be entirely coincidental." Additionally, the specific name of the type species means "leaf razor", which is most directly a reference to its keratin-covered jaws. Other distinguishing characteristics of Bulbasaurus among the geikiids include the hook-like beak, very large tusks, and absence of bossing on the prefrontal bone.
Other referred specimens include the nearly-complete skull CGP/1/938 (from the Wilgerbosch Kloof locality in Fraserburg), the complete skull CGP/1/949 (also from Wilgerbosch Kloof), the complete skull with associated lower jaws and postcranial remains CGP/1/970 (from the Blaauwkrans locality in Beaufort West, Western Cape), the complete but crushed skull CGP/1/2263 (locality unknown), the crushed skull with lower jaws SAM-PK-K10106 (from the Paalhuisberg locality in Beaufort West), and the complete juvenile skull with lower laws SAM-PK-K10587 (from the Doornhoek locality in Beaufort West). All of these specimens are either held at the Iziko Museums in Cape Town or the Council for Geoscience in Praetoria. Before being referred to the new genus Bulbasaurus, they were initially treated as specimens of Tropidostoma in collections.
Bulbasaurus was described by Christian Kammerer and Smith in 2017. The description states that the generic name combines the Latin bulbus, referring to the very large and bulbous nasal bone bosses, with the common suffix -saurus. As for the specific name phylloxyron, meaning literally "leaf razor", it is derived from the Greek and , and apparently refers to the keratinous covering on the premaxilla, maxilla, and palate that would have been used to shear plant material. Thus, as published, the name of Bulbasaurus does not directly refer to Pokémon, or specifically the similarly-named Bulbasaur. However, Kammerer noted that "if one wished to read between the lines concerning certain similarities, I wouldn't stop them", and later added that "similarities between this species and certain other squat, tusked quadrupeds may not be entirely coincidental."
Further back on the interior of the upper jaw, the are exposed as a palatine pad, which is very roughened and would have been covered in keratin, although the portion where they contact the premaxillae is smooth and sloping. The back portion of the palatines are thinner than the rest of the bone, but it is still thicker than that of either Tropidostoma or Oudenodon, instead resembling Aulacephalodon more closely. The are robust in contrast to Tropidostoma, and bear ridges that converge into a tall, blade-like process known as the crista oesophagea. The pterygoids also project outwards in rod-like structures to meet the .
Compared to other dicynodonts, the tusk of Bulbasaurus was massive; the holotype skull, which is long, has a tusk diameter. Only Aulacephalodon has comparably large tusks proportionally, but these belong to adult specimens much larger than Bulbasaurus (in juveniles the size of Bulbasaurus, the tusks are still erupting). The root of the tusk bulges outwards from the surface of the maxilla due to its large size. Extensive pitting on the surface of the maxilla is suggestive of some kind of keratinous covering, which has also been inferred for other dicynodonts. Unlike other geikiids and most other dicynodonts, there is no ridge behind the tusk, although mature Aulacephalodon also lack this ridge.
Typical of geikiids, the interorbital region between the eyes was quite broad. The , , frontal bones, and form the margin of the eye socket, with the portion comprised by the lacrimals having an orbital ridge that is better-developed and more raised. Unlike other cryptodontians, there is no evidence of a second set of bosses on the prefrontals, although their surfaces are somewhat thickened. A relatively deep midline depression (mildly developed in Tropidostoma and Oudenodon, but absent in other geikiids) is visible on the frontals, which are situated largely between the eyes and form roughened edges where they contributes to the rims of the eye sockets. There appears to be no separation of the postfrontal bones from the frontals, which is probably an adult characteristic as in Aulacephalodon. The elongate jugals form part of the zygoma, or bony cheek, and ends at the temporal fenestra. It also forms part of the temporal and postorbital bars; Pelanomodon differs from Bulbasaurus in having small bosses on the latter portions of the jugals.
As for the underlying parietals themselves, they are slightly concave. In front of the parietals are the small midline preparietal bones, which are relatively broad and have a rounded tip, as in Aulacephalodon and Pelanomodon but in contrast to Tropidostoma. The pineal gland is bordered by the preparietals and parietals, and it is surrounded by a simple ridge instead of being on a raised boss like either the large rhaciocephalids and Endothiodon or some large specimens belonging to Aulacephalodon. On the neurocranium, no sutures are visible, suggesting that the bones are very fused. The are likewise very fused. The contribution of the occipital bone to the back of the skull is unusually extensive and occupies much of the area not part of the squamosals above the level of the foramen magnum. Also unusual are the smaller elements at the back of the skull, namely the postparietals and tabulars. The postparietals are not part of the continuous flat surface at the back of the skull, instead forming a sharp divot; additionally, a strong midline crest is present on the postparietals and do not extend onto other bones. Meanwhile, the tabulars are wider than they are long.
These differences are most likely due to growth, as similar transformations are also seen in Aulacephalodon. However, the latter (and all other geikiids where the growth sequence is known) differs from Bulbasaurus in that the degree of overlap of the parietals by the postorbitals does not change; instead, the parietals themselves simply become wider. In this respect, Bulbasaurus retains the ancestral cryptodontian condition, which is also seen in rhachiocephalids as well as Oudenodon, Tropidostoma, and other oudenodontids. Overall, the relatively small Bulbasaurus provides evidence that the growth sequence of large geikiids such as Aulacephalodon did not develop along with their size, but rather was already present ancestrally and was retained as geikiids grew.
Tropidostoma, the genus Bulbasaurus was originally assigned to, exhibits two distinct morphologies - a robust morph with short snout and large tusks, and a gracile morph with long snout and small tusks, which probably represents sexual dimorphism as in other dicynodonts. However, Bulbasaurus matches neither of those morphologies; it differs from the genus Tropidostoma as a whole in many respects (addressed above). Additionally, even the holotype of Tropidostoma (which is probably immature judging by the unerupted tusks) is larger than adult specimens of Bulbasaurus, which further warrants their separation. The same is true of Bulbasaurus and Aulacephalodon, in addition to their differing boss morphologies and different interorbital widths. Bulbasaurus also differs from the problematic specimens BP/1/763 (assigned once to its own genus, Proaulacocephalodon, or to a juvenile Aulacephalodon) and TM 1480 (assigned once to Dicynodon) by its larger tusks and wider interorbital region, among other characteristics. These specimens are additionally from the younger Cistecephalus assemblage zone.
The Tropidostoma AZ also records the gradual diversification of and . Therocephalians present include Hofmeyria, Ictidosuchoides (most common), Ictidosuchops, Ictidosuchus, and Lycideops; gorgonopsids present include Aelurognathus, Aelurosaurus, Aloposaurus, Cyonosaurus, Gorgonops (most common), Lycaenops, and Scymnognathus. Within the Karoo Supergroup, also first appear within the Tropidostoma AZ; they include Abdalodon (formerly assigned to Procynosuchus) and Charassognathus. Rarer members of the Tropidostoma AZ assemblage include the burnetiamorpha Lobalopex and Lophorhinus; parareptilia Pareiasaurus and Saurorictus; the archosauromorpha Younginia; and the temnospondyli Rhinesuchus.
Bulbasaurus was probably buried on surrounding a meandering river up to wide and with up to in diameter. This river flowed from the southern mountains northeast onto an alluvial fan some wide. The water flow in the rivers was seasonally dependent, but there was probably flowing water year-round. About every 30,000 years, the river banks were breached by flooding, leaving overbank deposits and a series of small, isolated ponds.
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